Conn Organ Amp Conversion to Tweed Bassman

[Cygnus X1]

Conn organ amp is up and running.

I do these once in a while but I don't take much time to explain the process.

Obtain amp...free is good.
But you really don't want to pay much due to the fact that many organ amps, PA amps, and "other" tube amps may not have much that is usable for a guitar or bass amp.

First thing I try to do is get that amp-as is, up and running so I know what I have.
Sounds simple but it can really challenge the troubleshooting skills plus organ amps have so many other functions going on...it has to be divided up and conquered.
PA amps can also have everything including the kitchen sink thrown in, as if they are integrated mixing units with multiple impedance inputs and outputs designed to drive other power amps.

So I examined the Conn, determined it was safe to bring up with the variac while watching current draw (I really need to build a light bulb limiter...) which reforms the old canister caps.
M-fugg, this thing is LOUD for being a simple 2x6L6...but sounds good, no hum.

So now comes the hard part-is it close enough to convert?
Does it need to be gutted and rebuilt?
Or is it just good on its own?

Well time to sketch out what is in it, and research the brain and schems for
what fits.
I'll be gummed, looks REAL familiar.

Tweed bassman, but with a single 12AX7A instead of 12AY7's.
And cascaded already!
Cool, let's try it.
(This circuit is the origin of the Marshall JTM45, so hang in there...)

So I dig further into Leo's brain and I find a good fit-the Princeton followed a similar path.

So I installed jacks, volume and Princeton 5D2 style tone stack.
Sounds nice...and LOUD!

Turns out the only conversion is adding a few components to make it guitar-friendly
and cleaning up the stuff that I didn't need in there.





I see a problem with making a head cab though.
The power transformer is directly up against the chassis face.
No room for the cover!

Similar problem with placing the controls, too close to the output transformer so I had to squeeze
it all on one side.
The voicing is close enough to not have to mod the whole amp - a big plus.
Has a couple of blue molded caps in it...good stuff.



Also found out I have some Mullard 12AX7 pulls!
Along with a Mullard 5AR4/GZ34.
Cork sniffer tubes...I'll have to give those a try in the Cyg combo.